Service

VSR Seminar

Single magnetic skyrmions from first-principles: from pinning effects to electrical and x-ray reading

Speaker:

Prof. Dr. Samir Lounis, IAS-1

Contents:

Magnetic skyrmions are particles-like spin textures with rich physics and technological potential for future information and communication devices. Such magnetic entities have a topological nature providing a link between topology and spintronics. Their nucleation, motion and velocity are, however, heavily affected by materials inhomogeneities ubiquitous to any device. I will discuss first-time ab-initio based simulations of single magnetic skyrmions in Pd/Fe/Ir(111) surface interacting with 3d and 4d impurities and present an electronic-structure analysis to identify the important mechanisms behind the expulsion or pinning of single magnetic skyrmions as function of the chemical nature of the impurities. This translates on a bigger picture to an energy landscape with attractive and repulsive potentials determining the skyrmion motion. In contrast to usual theoretical investigations, we simulate fully ab-initio spin-textures containing up to a couple of hundreds atoms. I will also address first-principles inspired protocols to read magnetic skyrmions with electrical means via the tunneling spin-mixing resistance (TXMR), confirmed experimentally, or with soft x-ray spectroscopy. For instance, the TXMR effect can be used within a current perpendicular-to-plane geometry, which has immediate implications for device concepts. The impact of magnetic non-collinearity and spin-orbit interaction will be highlighted. Finally, I will demonstrate that by hosting ground state emergent magnetic fields, magnetic skyrmions can counter-intuitively carry a non-trivial orbital magnetization, even without the spin-orbit interaction. This means that the orbital moment contains a topological contribution that does not change under continuous deformations of the magnetic structure. This feature can be used in a new experimental protocol to identify topological magnetic structures. Finally, I will give an outlook for planned research works.

Studies of quantum dynamics in open and closed interacting systems are nowadays largely stimulated by the advent of novel materials and cold atomic gases and by the growing interest in technological areas like spintronics, quantum computing, and quantum information processing. Besides this more application-driven interest there persist fundamental and still unanswered questions related to the conditions under which a finite quantum system equilibrates and thermalizes.
Large-scale computer simulations are invaluable in studies of the manipulation and dynamics of quantum spin systems. For this purpose we use the Massively Parallel Quantum Dynamics Simulator, based on hybrid OpenMP/MPI code, which simulates the dynamics of quantum systems with up to 43 spin-1/2 particles on JUQUEEN. In essence, the simulation algorithm solves the time-dependent Schrödinger equation for a system of interacting, localized spin-1/2 particles without any restriction on the range of the interactions or on the dimensionality of the lattice. Models of this type are commonly used to describe a wide range of magnetic phenomena, (quantum) phase transitions, candidates for quantum computer hardware etc.
Some technical aspects of the Massively Parallel Quantum Dynamics Simulator and some of its applications in quantum computation and quantum statistical physics will be highlighted.